It refers to the movement of blood through a vessel, tissue, or organ.
It is generally expressed in terms of blood volume per unit of time.
It is started by the contraction of the heart’s ventricles. Ventricular contraction expels blood into the main arteries, causing a flow from regions of higher pressure to regions of lower pressure as the blood encounters smaller arteries and arterioles, then capillaries, then venules and veins of the system venous.
This section looks at a number of critical variables that contribute to blood flow throughout the body. It also looks at factors that impede or slow blood flow, a phenomenon known as resistance.
As noted above, hydrostatic pressure is the force exerted by a fluid due to gravitational pull, usually against the wall of the container in which it is located.
One form of hydrostatic pressure is blood pressure, the force exerted by blood on the walls of the blood vessels or chambers of the heart.
Blood pressure can be measured in the capillaries and veins, as well as in the vessels of the pulmonary circulation; however, the term “arterial pressure” without any specific descriptor typically refers to systemic arterial pressure, that is, the pressure of blood flowing in the arteries of the systemic circulation.
In clinical practice, this pressure is measured in mm Hg and is generally obtained using the brachial artery of the arm.
Blood pressure in the largest vessels consists of several different components: systolic and diastolic pressure, pulse pressure, and mean arterial pressure.
Systolic and diastolic pressures
When systemic blood pressure is measured, it is recorded as a ratio of two numbers (for example, 120/80 is normal blood pressure in adults), expressed as systolic pressure over diastolic pressure.
Systolic pressure is the highest value (typically around 120 mm Hg) and reflects the arterial pressure resulting from the expulsion of blood during ventricular contraction or systole.
Diastolic pressure is the lowest value (generally about 80 mm Hg) and represents the arterial pressure of the blood during ventricular relaxation or diastole.
The difference between systolic pressure and diastolic pressure is the pulse pressure. For example, an individual with a systolic pressure of 120 mm Hg and a diastolic pressure of 80 mm Hg would have a pulse pressure of 40 mm Hg.
In general, a pulse pressure should be at least 25 percent of the systolic pressure. A pulse pressure below this level is described as low or narrow.
This can occur, for example, in patients with a low-volume stroke, which can be seen in congestive heart failure, aortic valve stenosis, or significant blood loss after trauma.
In contrast, a high or wide pulse pressure is common in healthy people following strenuous exercise, when their resting pulse pressure of 30 to 40 mm Hg may temporarily increase to 100 mm Hg as the volume of the blow increases. .
A persistently high pulse pressure at or above 100 mm Hg can indicate excessive resistance in the arteries and can be caused by a variety of disorders.
Chronic high resting pulse pressures can degrade the heart, brain, and kidneys and warrant medical treatment.
Mean arterial pressure
Mean arterial pressure (MAP) represents the “average” pressure of the blood in the arteries, that is, the average force that drives the blood into the vessels that serve the tissues.
The mean is a statistical concept and is calculated by taking the sum of the values divided by the number of values.
After blood is expelled from the heart, elastic fibers in the arteries help maintain a high pressure gradient as they expand to accommodate blood and then recede.
This expanding and receding effect, known as the pulse, can be palpated manually or measured electronically.
Although the effect diminishes with distance from the heart, elements of the systolic and diastolic components of the pulse are still evident down to the level of the arterioles.
Because the pulse indicates the heart rate, it is measured clinically to provide clues to the health status of a patient. It is recorded as beats per minute. Both pulse rate and strength are clinically important.
A heart activity or other temporary factors can cause a high or irregular heart rate, but it can also indicate a heart condition. Pulse strength indicates strength of ventricular contraction and cardiac output.
If the pulse is strong, then the systolic pressure is high. If it is weak, the systolic pressure has dropped and medical intervention may be warranted.
The pulse can be palpated manually by placing the tips of the fingers on an artery that runs close to the surface of the body and pressing lightly.
While this procedure is typically performed using the radial artery in the wrist or the common carotid artery in the neck, any superficial artery that can be palpated can be used.
Common sites to find a pulse include the temporal and facial arteries in the head, the brachial arteries in the upper arm, the femoral arteries in the thigh, the popliteal arteries behind the knees, the posterior tibial arteries near the regions of the medial tarsus and the dorsal artery of the foot in the feet.
A variety of commercial electronic devices are also available to measure the pulse.
Blood pressure measurement
Blood pressure is one of the critical parameters measured in virtually all patients in every healthcare setting. The technique used today was developed more than 100 years ago by a pioneering Russian physician, Dr. Nikolai Korotkoff.
Turbulent blood flow through the vessels can be heard as a soft tic when measuring blood pressure; These sounds are known as Korotkoff sounds.
The blood pressure measurement technique requires the use of a sphygmomanometer (a blood pressure cuff connected to a measuring device) and a stethoscope.
Variables that affect blood flow and blood pressure
Five variables influence blood flow and blood pressure:
- Cardiac output.
- Volume of blood.
- Viscosity of blood.
- Length and diameter of blood vessels.
Remember that blood moves from a higher pressure to a lower pressure. It is pumped from the heart into the arteries under high pressure. If the pressure in the arteries increases (afterload) and the heart function does not compensate, the blood flow will decrease.
In the venous system, the opposite relationship is true. Increasing the pressure in the veins does not decrease the flow as it does in the arteries, but it actually increases the flow.
Since the pressure in the veins is usually relatively low, for blood to return to the heart, the pressure in the atria during atrial diastole must be even lower. It is normally close to zero, except when the atria contract.
Cardiac output is the measurement of blood flow from the heart through the ventricles, and is generally measured in liters per minute.
Any factor that causes increased cardiac output, raising heart rate or stroke volume, or both, will raise blood pressure and promote blood flow.
These factors include sympathetic stimulation, the catecholamines epinephrine and norepinephrine, thyroid hormones, and increased levels of calcium ions.
Conversely, any factor that decreases cardiac output, by decreasing heart rate or stroke volume, or both, will decrease blood pressure and blood flow.
These factors include parasympathetic stimulation, elevated or decreased levels of potassium ions, decreased levels of calcium, anoxia, and acidosis.
Compliance is the ability of any compartment to expand to accommodate more content. A metal tube, for example, is not supported, while a balloon is.
The higher the compliance of an artery, the more efficiently it can expand to accommodate surges in blood flow without increasing resistance or blood pressure. Veins are more compatible than arteries and can expand to hold more blood.
When vascular disease causes stiffness of the arteries, compliance is reduced and resistance to blood flow increases. The result is more turbulence, higher pressure within the vessel, and less blood flow. This increases the work of the heart.
A mathematical approach to the factors that affect blood flow
Jean Louis Marie Poiseuille was a French physician and physiologist who devised a mathematical equation that describes blood flow and its relationship to known parameters.
The same equation also applies to fluid flow engineering studies.
Although understanding the math behind the relationships between factors that affect blood flow is not necessary to understand blood flow, it can help solidify your understanding of their relationships.
Keep in mind that even if the equation seems intimidating, breaking it down into its components and following the relationships will make these relationships clearer, even if you are weak at math. Focus on the three critical variables: radius (r), vessel length (λ), and viscosity (η).
The relationship between blood volume, blood pressure, and blood flow is intuitively obvious. Water may simply trickle along a stream in a dry season, but run quickly and under great pressure after heavy rain.
Similarly, as blood volume decreases, pressure and flow decrease. As blood volume increases, pressure and flow increase.
Under normal circumstances, the blood volume varies little. Low blood volume, called hypovolemia, can be caused by bleeding, dehydration, vomiting, severe burns, or some medications used to treat hypertension.
It is important to recognize that other regulatory mechanisms in the body are so effective in maintaining blood pressure that an individual may be asymptomatic until 10-20 percent of blood volume is lost.
Treatment generally includes intravenous fluid replacement.
Hypervolemia, the excessive volume of fluid, can be caused by retention of water and sodium, as seen in patients with heart failure, liver cirrhosis, some forms of kidney disease, hyperaldosteronism, and some glucocorticoid steroid treatments.
Restoring homeostasis in these patients depends on reversing the condition that triggered the hypervolemia.
Viscosity is the thickness of fluids that affects their ability to flow. Clean water, for example, is less viscous than mud.
Viscosity of blood is directly proportional to resistance and inversely proportional to flow; therefore, any condition that causes viscosity to increase will also increase resistance and decrease flow.
For example, imagine drinking milk, then a milkshake, through the size of straw. You experience more resistance and therefore less flow from the shake.
Conversely, any condition that causes viscosity to drop (such as melting milk shake) will decrease resistance and increase flow.
Normally, the viscosity of blood does not change in short periods of time. The two main determinants of blood viscosity are formed elements and plasma proteins.
Since the vast majority of the elements formed are erythrocytes, any condition that affects erythropoiesis, such as polycythemia or anemia, can alter the viscosity.
Since most plasma proteins are produced by the liver, any condition that affects liver function can also change viscosity slightly and therefore decrease blood flow.
Liver abnormalities include:
- Damage from alcohol.
- Drug toxicity.
While leukocytes and platelets are normally a small component of the formed elements, there are some rare conditions in which severe overproduction can also affect viscosity.
Length and diameter of the container
The length of a vessel is directly proportional to its resistance: the longer the vessel, the greater the resistance and the lower the flow.
As with blood volume, this makes intuitive sense, as the increased surface area of the vessel will impede blood flow. Similarly, if the container is shortened, the resistance will decrease and the flow will increase.
The length of our blood vessels increases throughout childhood as we grow, of course, but it does not change in adults under normal physiological circumstances.
Furthermore, the distribution of the vessels is not the same in all tissues. Adipose tissue does not have an extensive vascular supply. One pound of adipose tissue contains approximately 200 miles of vessels, while skeletal muscle contains more than twice that.
In general, vessels decrease in length only during loss of mass or amputation.
An individual weighing 150 pounds has approximately 60,000 miles of glasses on the body. The approximately 10 pound gain increases from 2,000 to 4,000 vessel miles, depending on the nature of the tissue obtained.
One of the great benefits of weight reduction is the reduction of stress on the heart, which does not have to overcome the resistance of so many kilometers of vessels.
In contrast to length, the diameter of blood vessels changes throughout the body, depending on the type of vessel, as we saw earlier.
The diameter of any given vessel can also change frequently throughout the day in response to neuronal and chemical signals that trigger vasodilation and vasoconstriction.
The vascular tone of the vessel is the contractile state of smooth muscle and the primary determinant of diameter, and therefore resistance and flow.
The effect of vessel diameter on resistance is reversed: given the same volume of blood, a larger diameter means that there is less blood in contact with the vessel wall, reducing friction and resistance, increasing flow.
A reduced diameter means that more blood comes into contact with the wall of the vessel, and the resistance increases, which decreases the flow.
The influence of lumen diameter on resistance is dramatic: a slight increase or decrease in diameter causes a large decrease or increase in resistance.
This is because the resistance is inversely proportional to the radius of the blood vessel (half the diameter of the vessel) raised to the fourth power (R = 1 / r4).
This means, for example, that if an artery or arteriole contracts to half its original radius, the resistance to flow will increase 16 times. And if an artery or arteriole dilates to twice its initial radius, then the resistance in the vessel will decrease to 1/16 of its original value and the flow will increase 16 times.
The roles of vessel diameter and total area in blood flow and blood pressure
Remember that we classify arterioles as resistance vessels because, due to their small lumen, they drastically reduce the flow of blood from the arteries. In fact, the arterioles are the site of greatest resistance in the entire vascular network.
This may seem surprising, given that capillaries are smaller in size.
The pumping action of the heart pushes blood into the arteries, from an area of higher pressure to an area of lower pressure. If blood is to flow from the veins to the heart, the pressure in the veins must be greater than the pressure in the heart atria.
Two factors help maintain this pressure gradient between the veins and the heart. First, the pressure in the atria during diastole is very low, often close to zero when the atria are relaxed (atrial diastole).
Second, two physiological “pumps” increase the pressure in the venous system. The use of the term “pump” implies a physical device that accelerates the flow. These physiological pumps are less obvious.
Skeletal muscle pump
In many regions of the body, the pressure within the veins can be increased by the contraction of the surrounding skeletal muscle.
This mechanism, known as the skeletal muscle pump, helps lower pressure veins counteract the force of gravity, increasing pressure to move blood back to the heart.
When the leg muscles contract, for example when walking or running, they put pressure on nearby veins with their many one-way valves.
This increased pressure causes blood to flow upward, opening valves above the contracting muscles for blood to flow through.
Simultaneously, the valves below the contracted muscles close; therefore, the blood should not seep down to the feet.
Military recruits are trained to slightly flex their legs while standing at attention for extended periods. Otherwise, blood can pool in the lower extremities instead of returning to the heart.
Consequently, the brain will not receive enough oxygenated blood and the individual may lose consciousness.
The breathing pump helps blood flow through the veins in the chest and abdomen. During inhalation, the volume of the chest increases, largely through contraction of the diaphragm, which moves downward and compresses the abdominal cavity.
Elevation of the chest caused by contraction of the external intercostal muscles also contributes to the increased volume of the chest. The increase in volume causes the air pressure within the chest to decrease, allowing us to inhale.
Furthermore, as the air pressure within the chest decreases, the blood pressure in the thoracic veins also decreases, falling below the pressure in the abdominal veins.
This causes blood to flow along its pressure gradient from the veins outside the chest, where the pressure is higher, to the thoracic region, where the pressure is now lower. This in turn promotes the return of blood from the thoracic veins to the atria.
During exhalation, when air pressure increases within the chest cavity, the pressure in the thoracic veins increases, accelerating blood flow to the heart, while valves in the veins prevent blood from flowing backward from the veins. thoracic and abdominal.
Pressure relationships in the venous system
Although the diameter of the vessel increases from the smallest venules to the largest veins and eventually to the vena cavae (singular = vena cava), the total cross-sectional area actually decreases.
Individual veins are larger in diameter than venules, but their total number is much smaller, so their total cross-sectional area is also smaller.
Also note that as the blood moves from the venules to the veins, the average blood pressure decreases, but the speed of the blood actually increases. This pressure gradient pushes blood toward the heart.
Again, the presence of one-way valves and skeletal muscle and respiratory pumps contribute to this increased flow.
Since approximately 64 percent of the total blood volume resides in the systemic veins, any action that increases blood flow through the veins will increase venous return to the heart.
Maintaining vascular tone within the veins prevents the veins from simply stretching, moistening the blood flow, and as you will see, vasoconstriction actually improves the flow.
The role of venoconstriction in resistance, blood pressure, and flow
As discussed earlier, vasoconstriction of an artery or arteriole decreases radius, increasing resistance and pressure, but decreasing flow. On the other hand, venoconstriction has a very different outcome.
The walls of the veins are thin but irregular; thus, when the smooth muscle in those walls contracts, the lumen becomes more rounded.
The more rounded the lumen, the less surface area the blood will encounter and the less resistance the vessel will offer.
Vasoconstriction increases pressure within a vein as it does in an artery, but in veins, increased pressure increases flow.
Remember that the pressure in the atria, towards which venous blood will flow, is very low, approaching zero during at least part of the relaxation phase of the cardiac cycle.
Therefore, venoconstriction increases the return of blood to the heart.
Hemodynamics is the study of blood flow. It focuses on how the heart distributes or pumps blood throughout the body. The study of hemodynamics integrates several sciences, including biology, chemistry, and physics.
As the heart pumps blood through the blood vessels, it helps supply oxygen to the body’s organs and tissues.
This process is vitally important so that the body can support itself. Problems with the hemodynamic system can cause serious health problems, the most common of which is hypertension.
The hemodynamic system
Key elements of the hemodynamic system include heart rate, stroke volume, cardiac output, systemic vascular resistance, and blood pressure.
Heart rate, or pulse, is the number of times a heart beats in one minute. Stroke volume is the amount of blood pumped through a ventricle when it contracts.
Based on the pulse and stroke volume, we can calculate cardiac output, which is a measure of how much blood the heart (specifically, the left or right ventricle) can pump per unit of time.
It is calculated using the following formula:
Cardiac output = heart rate x systolic volume
The average stroke volume for humans is 75 ml per stroke. At that stroke volume, a heart that beats 70 times per minute will have a cardiac output roughly equivalent to the total volume of blood in the body.
Cardiac output is therefore a measure of how efficiently the heart can move blood throughout the body. In our normal daily activities, the production must be such that the body can distribute blood according to the demands that are imposed on it.
Exercise is a common example of the need for increased cardiac output.
Cardiac output is related to Ohm’s law. Ohm’s law states that the current through some conductor is proportional to the voltage across the resistance.
Similar to a circuit, the path of blood flow through the body is related to the resistance to flow exerted by the blood vessels.
Systemic vascular resistance is the resistance that the heart must overcome to successfully pump blood through the body. Cardiac output multiplied by systemic vascular resistance equals arterial pressure.
When cardiac output is affected (for example, due to heart failure), the body will have a hard time managing its daily needs. A decrease in cardiac output results in a decrease in the oxygen available to the body’s tissues and organs.
How to increase blood flow
Regular exercise is one of the most common and effective means of increasing blood flow. It is also important to stretch the body after sitting for long periods of time.
Simply getting up and walking for a few minutes after a long period of rest will help increase blood flow through the body.
The study of hemodynamics is of vital importance since the body needs oxygen to function. In medicine, hemodynamic monitoring is used to assess this relationship between the cardiovascular system and the oxygen needs of the body’s tissues.
These evaluations are designed to allow medical professionals to make appropriate decisions for their patients.
Similarly, when these evaluations indicate that a patient is having trouble meeting their own oxygen needs, they are classified as hemodynamically unstable.
These patients receive mechanical or pharmacological support so that they can maintain the necessary blood pressure and cardiac output.
Blood flow is the movement of blood through a vessel, tissue, or organ. The slowing or blocking of blood flow is called resistance. Blood pressure is the force that blood exerts on the walls of the blood vessels or chambers of the heart.
Components of blood pressure include systolic pressure, which results from ventricular contraction, and diastolic pressure, which results from ventricular relaxation.
Pulse pressure is the difference between systolic and diastolic measurements, and mean arterial pressure is the “average” pressure of the blood in the arterial system, which carries blood to the tissues. The pulse, the expansion and recession of an artery, reflects the heartbeat.
The variables that affect blood flow and blood pressure in the systemic circulation are: cardiac output, compliance, blood volume, blood viscosity, and the length and diameter of blood vessels.
In the arterial system, vasodilation and vasoconstriction of the arterioles are an important factor in systemic blood pressure: mild vasodilation greatly decreases resistance and increases flow, while vasoconstriction greatly increases resistance and decreases blood flow. flow.
In the arterial system, as resistance increases, blood pressure increases and flow decreases. In the venous system, constriction raises blood pressure as it does in arteries; The increased pressure helps return blood to the heart.
In addition, the constriction causes the lumen of the vessel to become more rounded, decreasing resistance and increasing blood flow.